1. Field of the Invention
The present invention relates to an ultrasonic probe using a piezoelectric polymer sheet, which is particularly effective for use with an ultrasonic diagnostic apparatus.
2. Brief Descpription of the Prior Art
Recently, diagnostic apparatus utilizing the ultrasonic pulse reflection method has come into wide use. This ultrasonic diagnosing apparatus uses a probe comprising a piezoelectric element. The scanning method of the probe is of mechanical type or of electronic type. A recent development on electronic scanning type ultrasonic diagnosing apparatus is remarkable.
It is known that an array prepared by dividing into a multiplicity of sub-divisions a piezoelectric ceramic element of lead titanate, lead titanozirconate or the like has heretofore been employed as a probe for use with apparatus of the electronic scanning type. This probe has a structure in which a number of piezoelectric unit elements 5a, 5b, 5c, . . . , 5n are bonded on a backing material 1 and disposed thereon with a predetermined distance separately or independently from each other, as shown in FIG. 1. Alternatively, as shown in FIG. 2, there is an alternative integrated probe embodiment in which only electrodes in the piezoelectric element are divided into small unit elements. Although this alternative integrated probe embodiment is simple enough to manufacture, the resulting ceramic element has a small attenuation of an ultrasonic wave in the element so that, for example, where an element portion corresponding to the electrode 3b is excited, the vibration may be propagated to the adjacent element portions, causing a crosstalk to occur. In usual instances where the piezoelectric element is electrically driven, the vibrations both in the thickness direction and in the transverse direction may occur. Accordingly, where the structure is of the integrated type as shown in FIG. 2, the adjacent elemment portions or unit elements are bound to each other by means of the vibration in the transverse direction so that the mode in the thickness vibration required is disturbed or distorted to a great extent. In order to prevent these drawbacks, it is necessary to separate each of the piezoelectric elements as shown in FIG. 1; however, such precision in processing and assembling is required that the cost of manufacture is excessive. Furthermore, as has already been described, an acoustic wave resulting from the lengthwise vibration is reflected against the side wall of the adjacent element, whereby noise acoustic waves are caused to occur.
Particular attention has now been drawn to piezoelectric polymer films represented by polyvinylidene fluoride for use as element of medical probes because their acoustic impedances are close to that of the living body.
The present invention has been completed as a result of extensive research and experiments carried out on the basis of a presumption that the separation of the elements as in FIG. 1 is not necessarily required in the case of piezoelectric polymer films by paying attention to the fact that the attenuation of ultrasonic waves in the piezoelectric polymer films is far larger than that of piezoelectric ceramic films and that piezoelectricity of polymer films varies in certain directions in the plane of the films with respect to others.